The authors have measured, by means of low-temperature photoluminescence spectroscopy, the dependence on the well thickness of the shift of the excitonic recombination in GaAs/GaAlAs quantum wells induced by an external electric field. With increasing thickness, up to 230 AA, an increasing Stark shift, amounting to more than 100 meV for an external field of approximately 105 V cm-1, has been found. A corresponding quenching of the luminescence intensity has also been measured. Theoretical calculations of the eigenstates have been carried out and shown to compare favourably with the experiments. Two different approaches, numerical solution of the Schrodinger equation and variational calculations of the eigenstates, yield similar results for the Stark shift. Using the latter method, the quenching of the photoluminescence is correlated with the decrease in the wavefunction overlap between electrons and holes, as they are spatially separated by the field. © 1987 IOP Publishing Ltd.